This invention pertains to apparatuses and methods of removing tail pipes when conducting downhole operations in boreholes which penetrate subterranean earth formations.
When drilling a borehole which penetrates one or more subterranean earth formations, it may be advantageous or necessary to create a hardened plug in the borehole. Such plugs are used for abandonment of the well, wellbore isolation, wellbore stability, or kick-off procedures. For instance, it is sometimes necessary to change the direction of the borehole as it is being drilled. In order to change direction, a harden mass of cement is often placed in the borehole in the vicinity of the location where the change in drilling direction is to begin. This hardened mass of cement is referred to in the art as a sidetrack plug or as a kickoff plug.
The specific function of a kickoff plug is to cause the drill bit to divert its direction. Accordingly, if the plug is harder than the adjacent formation, then the drill bit will tend to penetrate the formation rather than the plug and thereby produce a change in drilling direction. However, a kickoff plug may fail to cause the drill bit to change direction if the plug is unreasonably contaminated with a foreign material, such as drilling mud or fluid. Drilling fluid, when mixed in the unset cement, can render the set mass softer than the adjacent formation. Thus, extreme care and expense is usually taken to make sure that the drilling fluid does not mix with the cement plug.
Typically, a cement plug may be set in a borehole by pumping a volume of spacer fluid compatible with the drilling mud and cement slurry into the workstring. Then a predetermined volume of cement slurry is pumped behind the spacer fluid. The cement slurry travels down the workstring and exits into the wellbore to form the plug. The cement slurry typically exits through one or more openings located at the end of the workstring. In this context, the end of the workstring is usually referred to as the “tail pipe.” Drilling fluid is usually pumped behind cement slurry to maintain pressure within the workstring.
At this point, the workstring is raised within the wellbore to permit the entire volume of cement slurry inside the conduit to flow out of the bottom of the tail pipe. However, the tail pipe must be raised very slowly or the cement slurry and the drilling fluid will mix, which may destroy the integrity of the plug. The process of raising the tail pipe generally causes some damage to the plug because as the tail pipe is raised the drilling fluid in the workstring mixes with the cement slurry. What is needed therefore, is a method and apparatus to keep the drilling fluid in the tail pipe from mixing with the cement slurry as the tail pipe is removed.